Plasma electrolytic deposition (PED) is a process for electrolytically coating a conductive (metal) surface with a hard, glassy, corrosion-resistant protective layer such as a ceramic coating. The coating property and quality of the process is determined by many factors such as composition and concentration of the electrolytes, applied electrical voltage, current density and duration. Different names have been used for PED in the literature, including “plasma electrolytic oxidation (PEO)”, “plasma electrolytic saturation (PES)”, “plasma electrolytic nitriding/carburizing (PEN/PEC)”, “microarc oxidation (MAO)” or “spark anodizing”. A key feature of the process that differs from anodizing is the occurrence of plasma discharging at the metal-coating interface when employing high potentials. When the applied potential exceeds a certain critical breakdown point, a number of discrete short-lived microdischarges will appear and will be moving across the metal surface to form a surface film. This process can be used to grow ceramic coatings on metal substrate. The thickness of the surface coating could be in a range from tens of micrometers to hundreds of micrometers. Because these surface coatings can provide high hardness and a continuous barrier, they can offer protection against wear, corrosion or heat as well as electrical insulation.
The surface coating generated by this technique is actually a chemical conversion of the substrate metal into its oxide. During the microarcing process, the oxides grow both inwards and outwards from the original metal surface. Because of this conversion process, the coatings have strong adhesion to the substrate metal, comparing to conventional deposited coatings. A wide range of metal substrates can be coated, including aluminum alloys, zircoalloys, titanium alloys, magnesium alloys, and most cast alloys.
Through adjusting electrolyte composition, the metal surface can be saturated by the non-metallic elements such as O, C, N, B and the combination of these. These elements can form a vapor envelop along the metallic substrate surface and diffuse inward to the metal in a PED process. The diffusant species are the chemicals that can be negatively ionized in the electrolyte. The plasma will vaporize these species to form a vapor envelope. The electric field applied by PED will accelerate across the voltage drop to bombard the substrate surface through interstitial and grain-boundary diffusion. It was reported that PED could be used for nitriding, carburizing, boriding, carbonitriding and etc.
Another unique feature of PED is it can reach a temperature as high as 2×104° C. by a plasma thermo-chemical reaction in a time period of less than 10-6 s and then cooling down rapidly at a rate of 108 K/s. This feature enables PED to form some special surface structures such as metastable high temperature phases, nonequilibrium solid solutions, complex mixed-compounds, glassy glasses, etc. These special structures can be designed for anti-corrosion coating, super-hard surface protective coating, wear resistance coating, heat-protective coating, etc.
The prior art includes the following:
U.S. Pat. No. 6,806,613 discloses a process for plasma microarc oxidation for producing ceramic coatings on metal workpieces having semiconducting properties. Refer to FIG. 2. A current generator is disclosed that includes: a module for converting a sinusoidal AC periodic signal into a triangular or trapezoidal signal, the converting module having a power supply input; a module for modifying the slope and form factor of the voltage signal; a module to control variation in frequency; a module to control electrical energy to the DC output; and a micro-computer for managing the electrical energy. The voltage generator is connected to the anode immersed in an electrolytic bath.
U.S. Pat. No. 6,666,960 discloses an electroplating current supply system that includes a power supply unit for supplying an object to be plated with an electroplating current whose polarity is inverted at predetermined intervals. Refer to FIG. 2. The power supply unit includes a first DC power supply supplying a positive current and a second DC power supply supplying a negative current. These first and second DC power supplies are capable of producing DC power by rectifying commercial AC power. IGBT devices are utilized as high speed switching means and are operatively connected between the output of the respective DC power supplies to the load terminals coupled to the plating load.
The system also includes a processing unit for controlling the ratio in magnitude and duration of the positive current to the negative current supplied to the object so as to ensure uniform coating. A display device is connected to the processing unit to notify the operator that the circuit including the plating load has been opened (a safety feature for the operator).
U.S. Pat. Nos. 4,478,689; 4,517,059 (same assignee and disclosure) is an automated process for electrolytic processing of a metal surface, preferably by anodization. Of relevance, the process discloses the use of pre-programmable and computerized process. Specifically, a microprocessor for electronically monitoring voltages is disclosed. Refer to FIG. 3.
U.S. Pat. No. 5,049,246 is an apparatus for electrolytic processing such as electroplating is disclosed. A power supply for supplying time multiplexed power to electrodes is disclosed. The power supply may include a pulse width modulator or pulse position modulator and is operative to control the relative amounts of time that the respective electrodes are energized for electroplating.
Also the website http://en.wikipedia.orq/wiki/IGBT discloses a suggestion of using an IGBT with PWM modulation in medium and high power supplies. The relevant portions of the publication disclose the following: “The Insulated gate bipolar transistor or IGBT is a three-terminal power semiconductor device, noted for high efficiency and fast switching. It switches electric power in many modern appliances: electric cars, variable speed refrigerators, air-conditioners, and even stereo systems with digital amplifiers. Since it is designed to rapidly turn on and off, amplifiers that use it often synthesize complex waveforms with pulse width modulation and low-pass filters. The IGBT combines the simple gate-drive characteristics of the MOSFETs with the high-current and low-saturation-voltage capability of bipolar transistors by combining an isolated gate FET for the control input, and a bipolar power transistor as a switch, in a single device. The IGBT is used in medium- to high-power applications such as switched-mode power supply, traction motor control and induction heating” It is emphasized that the reference does not disclose the use of a power supply for electrolytic purposes having an integrated computer control module for controlled interruption of the PED arcing process. In fact the reference merely suggests that a power supply might utilize IGBTs and that IGBTs may be utilized in coordination with pulse width modulators. There is no suggestion here of how to manage such a power supply using an integrated control module or a further suggestion that the control module would provide for controlled interruption of a PED-like process.